Electronic devices can operate with low voltage direct current (DC) power. Sometimes the DC power can be sourced from a battery, especially in mobile or cordless electronic devices. Other times, electronic devices can be powered with an alternating current (AC) power supply that can produce one or more DC voltages for device operation. Often times, electrical efficiency can be an important performance metric for electronic devices. Electrical efficiency can be a measure of the overall efficiency of a power supply with respect to converting an AC line voltage to the one or more DC voltages. Some electronic devices use switching power supplies because they can be relatively compact and relatively power efficient, especially when operated from about one half to full output capacity. For example, a 500 watt switching power supply may operate at 87% efficiency when supply from 250 to 500 watts. Other times, however, when operating at low output levels such as 10 watts, regular switching power supply efficiency can drop to lower levels (75%, for example).
To help efficiency performance during low load operation, some switching power supplies offer a burst-mode of operation. During burst-mode, the switching power supply can function for “bursts” of time. A switching power supply can switch an output transistor at a fairly high frequency such as 250 KHz. During burst-mode, the power supply can alter switching frequencies for bursts of time, effectively changing (slowing down) the switching rate. This can result in more output droop, but can increase overall efficiency since the power supply is not running continuously. Because power supply burst-mode operation is only applicable when the power supply is operating at relatively low current loads, operational mode selection is typically performed by the power supply. The power supply can measure the supplied load current and automatically make the mode determination without any interaction from the load. However to keep power supply costs low, current load sensing may not be too sensitive. As a result, burst-mode can be enabled at widely variable current loads. Further, the current sensing and amplification circuitry can consume power when they operate, which can add to a power loss overhead for the power supply, especially at light loads.
One disadvantage of burst-mode power supply operation is that the power supply can exhibit increased acoustic noise. Under normal operation, a switching power supply can operate at relatively high frequencies, well above the range of human hearing, such as 250 KHz for example. Operating in burst-mode, the switching frequency can effectively become lower, even below 20 KHz, where power supply operation can become audible. Audible noise can come from power supply components, such as ceramic capacitors, that emit noise as a signal frequency enters a range typically sensitive to human hearing. Some burst-mode capable power supplies can change switching frequencies in response to output current. Acoustic noise from the power supply can be frequency dependent; and as such, different switching frequencies can be acoustically noisier than others. For example, some users can choose to charge accessories (cell phones, for example) through a computing device. When charging while the computing device is in a low power state, the power supply can operate in burst-mode, but can change switching frequencies as the accessory charges and thereby produce varying acoustic noise. In some cases, the strength of the audible noise can be directly proportional to a load level seen by the power supply as the power supply enters burst mode.